Decades of research have established the renin-angiotensin system as a major regulator of blood pressure, homeostasis, and cell proliferation. Renin-angiotensin system effects are mediated by the opposing actions of the peptides angiotensin II (Ang II), a potent vasoconstrictor and mitogen, and angiotensin-(1-7) [Ang-(1- 7)], a vasodilator and growth inhibitor. Angiotensin-converting enzyme 2 (ACE2) recently was identified as a homologue of ACE that preferentially forms Ang-(1-7) from Ang II. Reduced ACE2 expression in the kidney of hypertensive rats suggests its contribution to the balance of Ang II to Ang-(1-7). Although ACE2 was initially localized in peripheral organs such as heart, kidney and testis, we identified ACE2 mRNA and protein in distinct brain regions from both neonatal and adult Sprague-Dawley rats as well as cultured neurons and astrocytes. Further, we demonstrated that treatment of cultured neurons and astrocytes with Ang II caused a marked reduction in ACE2 mRNA and protein. In exciting new results, we showed that inhibition of ACE2 reduced baroreflex control of heart rate, suggesting that the regulation of ACE2 and alterations in the ratio of Ang II to Ang-(1-7) is critical in central control of blood pressure. We propose that ACE2 serves as a major regulator of the balance of Ang ll/Ang-(1-7) concentration in the brain. In Specific Aim 1, we will ascertain whether Ang II regulates ACE2 mRNA and protein in the brain of transgenic rats expressing altered peptide concentrations and whether baroreflex control of heart rate is altered in these rats or following inhibition of ACE2 activity. Studies in Specific Aim 2 will focus on ACE2 secreted from cultured astrocytes, to assess its substrate specificity and determine the molecular mechanism of ACE2 secretion from cultured astrocytes and neurons and from intact brain slices. Finally, in Specific Aim 3, we will identify the transcriptional regulators of ACE2 in astrocytes and neurons. Relevance: ACE2 preferentially converts Ang II to Ang-(1-7), peptides which have opposing actions in the central control of blood pressure. The regulation of ACE2 may be critical to maintaining the normal function of the renin-angiotensin system in the brain.